There are known a number of photosensitive materials comprising a photosensitive layer on a support wherein images are formed by imagewise exposure. Among these, a technique of forming images through heat development is known as a system capable of simplifying image forming means and contributing to the environmental protection.
From the contemporary standpoints of environmental protection and space saving, it is strongly desired in the medical imaging field to reduce the quantity of spent solution. Needed in this regard is a technology relating to thermographic photosensitive materials for use in medical diagnosis and general photography which can be effectively exposed by means of laser image setters and laser imagers and produce distinct black images having high resolution and sharpness. These thermographic photosensitive materials offer to the customer a simple thermographic system which eliminates a need for solution type chemical agents and is not detrimental to the environment.
Photothermographic materials which are processed by a photothermographic process to form photographic images are disclosed, for example, in U.S. Pat. Nos. 3,152,904 and 3,457,075, D. Morgan and B. Shely, "Thermally Processed Silver Systems" in "Imaging Processes and Materials," Neblette, 8th Ed., Sturge, V. Walworth and A. Shepp Ed., page 2, 1969. These photothermographic materials generally contain a reducible silver source (e.g., organic silver salt), a catalytic amount of a photocatalyst (e.g., silver halide), and a reducing agent, typically dispersed in a binder matrix. Photothermographic materials are stable at room temperature. When they are heated at an elevated temperature (e.g., 80.degree. C. or higher) after exposure, redox reaction takes place between the reducible silver source (functioning as an oxidizing agent) and the reducing agent to form silver. This redox reaction is promoted by the catalysis of a latent image produced by exposure. Silver formed by reaction of the reducible silver salt in exposed regions provides black images in contrast to unexposed regions, eventually forming an image.
Photothermographic materials of this type are well known and most of them have a photosensitive layer which is formed by coating a coating solution in an organic solvent such as toluene, methyl ethyl ketone and methanol. The use of organic solvents is hazardous to workers involved in the manufacturing process and disadvantageous because of an extra cost for solvent recovery.
It was devised to form a photosensitive layer using a coating solution of water solvent (sometimes referred to as aqueous photosensitive layer) without such concern. For example, JP-A 52626/1974 and 116144/1978 disclose the use of gelatin as a binder. JP-A 151138/1975 discloses the use of polyvinyl alcohol as a binder. JP-A 61747/1985 discloses the combined use of gelatin and polyvinyl alcohol. JP-A 28737/1983 discloses a photosensitive layer containing water-soluble polyvinyl acetal as a binder.
The use of these binders leads to environmental and economical benefits because a photosensitive layer can be formed using a coating solution in water solvent.
The use of gelatin, polyvinyl alcohol, polyacetal and other water-soluble polymers as the binder, however, results in photosensitive materials which are of extremely low commodity worth in that a coating whose surface quality is practically acceptable is not available since these polymers are less compatible with the organic silver salt, that the silver tone of developed areas becomes brown or yellow and far from the essentially favorable black and that exposed areas have a low blackened density and unexposed areas have a high density.
There is a desire to develop a photothermographic material or aqueous photosensitive material having environmental and economic benefits, good coating surface quality, acceptable silver tone and satisfactory photographic properties upon development.